When you work as an employee, your employer must withhold money that it expects you to owe in Federal Insurance Contributions Act taxes, which include the Medicare tax and the Social Security tax, plus federal income taxes. Knowing how your employer figures this amount helps you budget for the amount of your paycheck that you will not receive. In addition to needing to know your salary and pay period, you also need to know how many personal allowances you claimed on your W-4 form, which you can find out by asking your employer.

Multiply the salary by the Medicare tax rate. As of 2011, the Medicare rate equals 0.0145. For example, if your semi-monthly salary equals $1,710, multiply $1,710 by 0.0145 to get $24.80.

Multiply the salary by the Social Security tax rate. For 2011 only, the rate drops to 4.2 percent from the typically 6.2 percent rate. In this example, multiply 0.042 by $1,710 to find $71.82 will be withheld for Social Security taxes.

Locate your withholding table in IRS Publication 15 for your withholding table based on your pay period and whether you are married or single. In this example, if you are single, use the single semi-monthly table. As of 2011, these tables start on page 38.

Find your salary range on the left-most column and your number of allowances on the top row. Where the salary range row and allowances column intersect, that is the amount of money withheld for your federal income taxes. In this example, if you claim four allowances, find where the $1,700 to $1,720 row intersects with the four allowances column to find that $133 would be withheld.